1. Field of the Invention
This invention relates to polypropylene compositions and more particularly pertains to a novel polypropylene impact blend having good mechanical properties along with significantly improved optical properties.
2. Description of the Prior Art
Polypropylene impact blends comprised of polypropylene modified by the addition of certain types of amorphous ethylene-propylene copolymer elastomers and certain types of polyethylenes are well-known in the art. As known the addition of certain types of these polymeric components in minor amounts to polypropylene results in polypropylene impact blends having good overall mechanical property balances, particularly improved impact strengths, as compared to unmodified polypropylenes. Such polypropylene impact blends are thus particularly useful in the manufacture of molded and extruded shaped articles, such as containers, filament rods, and the like, in accordance with conventional molding and/or extrusion techniques.
By way of example, U.S. Pat. No. 3,256,367 discloses a modified polypropylene impact blend having improved impact strength, stiffness and heat resistance properties as compared to unmodified polypropylene or other known polypropylene blends modified with minor amounts of rubber products, such as the above-mentioned ethylene-propylene copolymer elastomers, polyisobutylene, and the like. As described in the patent, the blends are comprised of 50 to 96% by weight polypropylene, 2 to 25% by weight of certain polyethylenes, particularly high density polyethylenes, as exemplified, and 2 to 25% by weight of certain types of amorphous ethylene/propylene copolymer or polyisobutylene elastomers.
Notwithstanding the overall improvements which have been obtained by the incorporation of such additives, such improvements have been achieved at the expense of producing polypropylene impact blends having extremely poor optical properties. More specifically, such blends are essentially opaque and, accordingly, cannot be employed in the manufacture of products requiring clarity.
U.S. Pat. No. 3,515,775 discloses a polypropylene blend of certain types of polypropylene, high density crystalline polyethylene and an amorphous ethylene-propylene copolymer which has relatively high optical properties for the manufacture of clear films and sheets. However, the types of polymer components employed respectively have relatively low molecular weights and, as disclosed, are useful only in the manufacture of such films and sheets.
So far as is presently known, no one has previously produced a polypropylene impact blend having both good mechanical properties, particularly high impact strength, stiffness, heat distortion resistance, etc., along with acceptable optical properties for use in the manufacture of high clarity articles in accordance with conventional injection molding, blow molding, and/or extrusion techniques.